Christopher Tolkien notes that this text was not known to him when the Silmarillion was prepared for publication, and also that '... it is so closely based on the rewritten form of the Lay as to read in places almost as a prose paraphrase of the verse.' Christopher Tolkien, commentary, The Lost Road And Other Writings

And as you say, although a substantial text it goes no further than the betrayal by Dairon to Thingol of Beren's presence in Doriath.